Reject unexpected MCP read tool arguments

app/mcp_tools.py

@@ -126,6 +126,11 @@ def optional_bool_arg(field: str, default: bool = False) -> bool:
             raise ValueError(f"{field} must be a boolean")
         return value
 
+    def reject_unexpected_args(allowed: set[str]) -> None:
+        unexpected = sorted(set(args) - allowed)
+        if unexpected:
+            raise ValueError(f"unexpected argument: {unexpected[0]}")
+
     def bounty_by_issue_number(repo_selector: str | None) -> Bounty | None:
         issue_query = select(Bounty).where(Bounty.issue_number == positive_int_arg("issue_number"))
         if repo_selector is not None:
@@ -193,6 +198,7 @@ def selected_bounty(internal_id_field: str) -> Bounty | None:
             )
             return json.dumps(sorted_bounties[:limit])
         if name == "get_bounty":
+            reject_unexpected_args({"id", "include_awards", "issue_number", "repo"})
             bounty = selected_bounty("id")
             if bounty is None:
                 return "bounty not found"
@@ -201,6 +207,9 @@ def selected_bounty(internal_id_field: str) -> Bounty | None:
                 bounty_data["awards"] = bounty_awards_to_dict(session, bounty.id)
             return json.dumps(bounty_data)
         if name == "list_bounty_attempts":
+            reject_unexpected_args(
+                {"bounty_id", "include_expired", "issue_number", "limit", "repo"}
+            )
             bounty = selected_bounty("bounty_id")
             if bounty is None:
                 return "bounty not found"
@@ -218,6 +227,7 @@ def selected_bounty(internal_id_field: str) -> Bounty | None:
                 "attempts": attempt_listing["attempts"],
             }
         if name == "get_balance":
+            reject_unexpected_args({"account"})
             account = normalized_account(str_arg("account"))
             return f"{account}: {format_mrwk(get_balance(session, account))} MRWK"
         if name == "register_wallet":
@@ -228,6 +238,7 @@ def selected_bounty(internal_id_field: str) -> Bounty | None:
             )
             return json.dumps(wallet_to_dict(session, wallet))
         if name == "get_wallet":
+            reject_unexpected_args({"address"})
             wallet_row = session.get(Wallet, normalized_wallet_address(str_arg("address")))
             if wallet_row is None:
                 return "wallet not found"
@@ -244,11 +255,13 @@ def selected_bounty(internal_id_field: str) -> Bounty | None:
             )
             return json.dumps(wallet_transfer_to_dict(transfer))
         if name == "get_ledger_entry":
+            reject_unexpected_args({"sequence"})
             entry = ledger_entry_to_dict(session, positive_int_arg("sequence"))
             if entry is None:
                 return "ledger entry not found"
             return json.dumps(entry)
         if name == "get_proof":
+            reject_unexpected_args({"hash"})
             proof = session.get(Proof, proof_hash_from_path(str_arg("hash")))
             if proof is None:
                 return "proof not found"

tests/test_api_mcp.py

@@ -814,6 +814,44 @@ def test_mcp_rejects_unknown_tool_name(sqlite_url: str) -> None:
     }
 
 
+@pytest.mark.parametrize(
+    ("tool_name", "arguments", "request_id"),
+    [
+        ("get_bounty", {"id": 1, "bounty": 1}, 13),
+        ("list_bounty_attempts", {"bounty_id": 1, "attempt_status": "active"}, 14),
+        ("get_balance", {"account": "treasury:mrwk", "acount": "typo"}, 15),
+        ("get_wallet", {"address": "mrwk1" + ("a" * 40), "wallet": "typo"}, 16),
+        ("get_ledger_entry", {"sequence": 1, "seq": 2}, 17),
+        ("get_proof", {"hash": "a" * 64, "proof": "typo"}, 18),
+    ],
+)
+def test_mcp_read_tools_reject_unexpected_arguments(
+    sqlite_url: str, tool_name: str, arguments: dict[str, object], request_id: int
+) -> None:
+    create_schema(sqlite_url)
+    with session_scope(sqlite_url) as session:
+        ensure_genesis(session)
+
+    client = TestClient(create_app(database_url=sqlite_url, webhook_secret="secret"))
+
+    response = client.post(
+        "/mcp",
+        json={
+            "jsonrpc": "2.0",
+            "id": request_id,
+            "method": "tools/call",
+            "params": {"name": tool_name, "arguments": arguments},
+        },
+    )
+
+    assert response.status_code == 200
+    assert response.json() == {
+        "jsonrpc": "2.0",
+        "id": request_id,
+        "error": {"code": -32602, "message": "invalid tool arguments"},
+    }
+
+
 def test_mcp_get_bounty_can_include_accepted_awards(sqlite_url: str) -> None:
     create_schema(sqlite_url)
     with session_scope(sqlite_url) as session:

tests/test_mcp_tools.py

@@ -83,6 +83,28 @@ def test_call_mcp_tool_rejects_c1_status_before_normalizing(sqlite_url: str) ->
         call_mcp_tool(sqlite_url, "list_bounties", {"status": "\u0085open"})
 
 
+@pytest.mark.parametrize(
+    ("tool_name", "arguments", "unexpected"),
+    [
+        ("get_bounty", {"id": 1, "bounty": 1}, "bounty"),
+        ("list_bounty_attempts", {"bounty_id": 1, "attempt_status": "active"}, "attempt_status"),
+        ("get_balance", {"account": "treasury:mrwk", "acount": "typo"}, "acount"),
+        ("get_wallet", {"address": "mrwk1" + ("a" * 40), "wallet": "typo"}, "wallet"),
+        ("get_ledger_entry", {"sequence": 1, "seq": 2}, "seq"),
+        ("get_proof", {"hash": "a" * 64, "proof": "typo"}, "proof"),
+    ],
+)
+def test_call_mcp_tool_rejects_unexpected_read_tool_arguments(
+    sqlite_url: str, tool_name: str, arguments: dict[str, object], unexpected: str
+) -> None:
+    create_schema(sqlite_url)
+    with session_scope(sqlite_url) as session:
+        ensure_genesis(session)
+
+    with pytest.raises(ValueError, match=f"unexpected argument: {unexpected}"):
+        call_mcp_tool(sqlite_url, tool_name, arguments)
+
+
 def test_call_mcp_tool_rejects_c1_nonce_before_integer_parsing(sqlite_url: str) -> None:
     create_schema(sqlite_url)
     with session_scope(sqlite_url) as session: